1. Field of the Invention
The present invention relates to an improvement in a tightening tool such as an impact wrench or an impact screwdriver.
2. Description of the Prior Art
A tightening tool such as an impact wrench or an impact screwdriver is often used to firmly tighten a threaded object such as a bolt or a nut. The tightening tool includes a hammer rotatably driven by a drive source such as an electric motor or an air motor. Further, the tightening tool includes an anvil which serves to engage the object to be tightened for rotating the same. The hammer and the anvil are interlocked with each other in such a manner that the hammer rotates the anvil through an impact applied thereto but the hammer becomes idle relative to the anvil when a load more than a predetermined value has been applied from the hammer to the anvil. As long as the object is driven into a work by a relatively smaller load, the anvil is continuously rotated by the hammer, and therefore, the object is continuously driven. When the object has been tightened to the effect that the load applied between the anvil and the hammer exceeds the predetermined value, the hammer becomes idle but again impacts the anvil after it has been rotated by a predetermined angle. Thus, the hammer repeatedly becomes idle and impacts on the anvil. The anvil is rotated for each impact by the hammer, and the object is tightened for each rotation of the anvil. This kind of tightening tool is disclosed in Japanese Laid-Open Utility Model Publication No. 2-19476 and many other publications.
In this kind of tightening tool, a resulted tightening torque of the object depends on the number of impact by the hammer. Therefore, to tighten the object by a strong force, the hammer is rotated to impact the anvil frequently. This means that the tightening torque can be adjusted by adjusting the number of impact.
Japanese Patent Publication No. 51-43240 in the name of the same assignee as the present application discloses an improved technique for adjusting the tightening torque. In this technique, the tightening torque is adjusted by adjusting the time during rotation of a hammer without directly detecting the frequency of impact. This technique may effectively operate to drive objects such as screws of the same standard into threaded holes of the same standard to the effect that the objects are tightened by substantially constant torque. However, in case that the objects are those which can be tightened by relatively low torque and that the time required for driving the objects varies with the objects, a constant tightening torque may not be obtained.
Japanese Utility Model Publication No. 53-21836 discloses a technique to directly detect the frequency of impact. This technique has been developed in view of the fact that a hammer is retracted away from an anvil along the rotational axis of the hammer for each idle rotation of the hammer relative to the anvil. A proximity switch is disposed adjacent the retracted position of the hammer so as to count the frequency of retraction (which is equal to the frequency of impact) of the hammer. When the counted frequency reaches a predetermined number, a drive source (an electric motor) is stopped to drive the hammer.
The inventor of the present invention has carried out an experiment to see how the technique disclosed in Japanese Utility Model Publication No. 53-21836 operates. Thus, the inventor has disposed a proximity switch adjacent a retracted position of a hammer and has counted the frequency of retracting movement of the hammer. As a result of this, the inventor has found that the counted frequency has tendency not to exactly correspond to actual frequency and that constant tightening torque cannot be obtained. The inventor conjectures the reason of such incorrect counting as follows. Since the anvil and the hammer are repeatedly rotated at high speeds, grease is filled around the hammer for the purpose of lubrication. A certain kind of metal in the form of fine powder is normally dispersed in the grease for improving the lubricity. This means that the metal powder exists around the proximity switch. Because of existence of such metal powder, the proximity switch malfunctions to the effect that the frequency of the retracting movement cannot be correctly counted.
Further, since the retracting position of the hammer is normally at a forward end portion of a tightening tool, the technique of Japanese Utility Model Publication No. 53-21836 involves another problem that the number of process for manufacturing a tightening tool may increase and that the proximity switch tends to pick up noises.